Lemma 2.1 ([1]}) 
Let \(G=(V,E)\)  be a \(d\) -regular \(\lambda \)  one-sided expander. Let \(T\subseteq V\)  be such that the graph induced on \(T\) , denoted \(G(T)\) , has average degree at least \(\delta d\) . Then \(|T| \geqslant (\delta - \lambda )\cdot |V|\) , and the number of edges in \(G(T)\)  is at least \((\delta - \lambda )\delta \cdot |E|\) .

